Endogenous type C virus expression in the rat myogenic cell line, L8, has been characterized. Release of virus was measured during the proliferative and differentiative phase of cultivation. Virus production was unaffected by blocking terminal diferentiation but was diminished by inhibitors of DNA syntheses. Exogenous infection of L8 cells with several helper independent type C viruses has been shown to block myogenic differentiation. There was an excellent correlation between inhibition of morphological and biochemical differentiation.